Allies of Boris Johnson have launched an all-out effort to scupper a parliamentary inquiry into Partygate after the senior official who led an initial inquiry into the scandal was unexpectedly unveiled as Keir Starmer’s new chief of staff.

The hire is a major coup for Starmer, who has been looking to appoint a veteran civil servant to prepare the party for government.

But in a furious response, a friend of Johnson said the decision of Sue Gray to work for Labour, 10 months after her report into gatherings in and around Downing Street during lockdown greatly undermined him, called her findings into doubt.

This, in turn, the friend said, meant the cross-party privileges inquiry of MPs into whether Johnson misled parliament was now in doubt, on the basis that it was led by Gray’s information.

“The privileges committee have taken to Sue Gray’s report as the fundamental basis of their investigation, and they have got Sue Gray’s unredacted evidence,” they told the Guardian.

“So I don’t see how if the privileges committee is going to proceed, how it can use Sue Gray’s findings, knowing that the orchestrator and investigator of those findings is now Keir Starmer’s chief of staff. Labour have scuppered the key evidential core of that investigation.”

The investigation was set up by a Commons vote, however, and can only be disbanded in the same way, making such a move unlikely.

Johnson has previously sought to undermine the inquiry by releasing a legal opinion calling into question its remit and validity, at a public cost of almost £130,000.

In a move that caused shock in Westminster, Gray was unveiled on Thursday as Starmer’s choice for his new chief of staff. The veteran official, who has led numerous investigations into ministerial misconduct and served for decades in Whitehall departments, has resigned from her civil service role.

Labour said she had been offered the role and hoped to accept it in line with advice from the appointments watchdog. The news has drawn a furious response from Conservatives, with one Whitehall official saying No 10 was “gobsmacked”.

A party spokesperson said: “The Labour party has offered Sue Gray the role of chief of staff to the leader of the opposition. We understand she hopes to accept the role subject to the normal procedures. Keir Starmer is delighted she is hoping to join our preparations for government and our mission to build a better Britain.”

Confirming Gray’s resignation, a Cabinet Office spokesperson said they were “reviewing the circumstances under which she resigned” and the department is understood to be making inquiries into whether she accepted an appointment before informing the appointments watchdog.

Labour said Gray’s acceptance was still subject to approval by the Advisory Committee on Business Appointments, which can advise gaps of several months on senior ex-ministers or civil servants who want to take up new jobs.

A senior Labour official said there were similar hirings by the Tories in the run-up to the 2010 election, including the Treasury’s James Sassoon and ex-army chief Sir Richard Dannatt.

The appointment prompted a furious reaction from some Conservative MPs and threatens to reignite months of arguments about the scandal.

On Thursday evening, Jacob Rees-Mogg called for an inquiry into Gray’s contacts with the Labour party, and said her decision to to take a job with Labour “invalidates” her report into lockdown parties in Downing Street.

Speaking on GB News, the former business secretary said: “It is hard not to feel that she has been rewarded and offered a plum job for effectively destroying a prime minister and creating a coup.

“This blows apart the idea of civil service impartiality … this appointment invalidates her Partygate report and shows that there was a socialist cabal of Boris haters who were delighted to remove him.”

Bassetlaw MP Brendan Clarke-Smith said he was “genuinely shocked” and accused
Starmer of having “scant regard for the public image of the civil service and the damage this will do”.

The move, the friend of Johnson argued, “brings into question the entire foundation of the Partygate story”.

They added: “For us, it really does cause a lot of doubts on Sue Gray’s report and the process that she led. I think there are some key questions that anybody would want answered – when did she begin to have these discussions with Labour?

“The politics of this are very, very difficult for the civil service clearly because she had an incredibly decisive role in putting together the process which ultimately led to the prime minister’s resignation. And now she is Keir Starmer’s most senior political appointee.”

The theory of a plot is arguably diminished by the fact that Gray was appointed to lead the inquiry by Johnson’s No 10, after the cabinet secretary, Simon Case, recused himself over claims he had attended some gatherings. There has also been nothing to indicate the events detailed in Gray’s published report were not accurate.

Starmer axed his previous chief of staff, Sam White, last year, moving key parts of his operation to Labour headquarters under the direction of his close ally Morgan McSweeney, saying the party was now on an election footing.

Starmer was said to have been seeking a chief of staff in the mould of Tony Blair’s recruitment of the diplomat Jonathan Powell to help prepare a team and shadow cabinet that are relatively inexperienced when it comes to government.

Powell on Thursday evening said Gray’s appointment was “another sign that Labour is ready to take over the reins of power from an exhausted and fractured Conservative party”.

Others who have been rumoured to be in Starmer’s sights include the former Treasury permanent secretary Tom Scholar and the former Brexit negotiator Olly Robbins.